What is the true space group of weberite?

Title What is the true space group of weberite? Author: Knop, Osvald; Cameron, T.Stanley & Jochem, KlausJournal of Solid State Chemistry, 1982 1982http://www.sciencedirect.com/science/article/pii/0022459682902316More details: "Analysis of X-ray diffraction evidence obtained from a single crystal of natural weberite, Na2MgAlF7, at room temperature and at ?��140??C rules out all space groups but two, the noncentrosymmetric Imm2 (originally proposed by Bystr\"{o}m) and I212121. Howevercomparison of structure refinements in these two groups and in Imma shows that the departure from centrosymmetry is so slightand the positional and thermal parameters of some of the F atoms in the Imm2 and \{I212121\} refinements are so highly correlatedthat the descriptions of the weberite structure in the three space groups must be regarded as practically indistinguishable. In the absence of a proof of achirality Bystr\"{o}m's space group assignment is provisionally accepted as valid, and Na2MgAlF7 is considered as isostructural with the recently refined Na2NiFeF7. "